Imidazole based nitrogen-sulfur ligands useful in radiographic imaging agents

ABSTRACT

The present invention relates particularly to novel imidazole based nitrogen-sulfur ligands that are suitable for complexing with a radionuclide, and are useful as general imaging agents for diagnostic purposes.

This is a divisional of application Ser. No. 08/030,940 filed Mar. 12,1993, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to novel ligands for forming radionuclidecomplexes, new complexes incorporating such ligands, processes forpreparing such complexes, imaging agents incorporating such complexes,and methods of imaging using such imaging agents.

The use of radiographic imaging agents for visualizing skeletalstructures, organs, or tissues, is well known in the area of biologicaland medical research and diagnostic procedures. The procedure wherebysuch imaging is accomplished, generally involves the preparation ofradioactive agents, which, when introduced to the biological subject,are localized in the specific skeletal structures, organs or tissues tobe studied. The localized radioactive agents may then be traced, plottedor scintiphotographed by radiation detectors, such as, traversingscanners or scintillation cameras. The distribution and relativeintensity of the detected radioactive agents indicates the position ofthe tissue in which the agent is localized, and also shows the presenceof aberrations, pathological conditions or the like.

In general, the radiographic imaging agents compriseradionuclide-labelled compounds; such as complexes of technetium 99m,rhenium 186 or rhenium 188, or other applicable radionuclides; withappropriate carriers, and auxiliary agents, such as delivery vehiclessuitable for injection into, or aspiration by, the patient,physiological buffers and salts, and the like.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates particularly to novel aminothiol ligandsthat are suitable for complexing with a radionuclide, and are useful asgeneral imaging agents for diagnostic purposes. In particular thepresent invention relates to novel ligands having the general formula:##STR1## wherein R¹ is selected from the group consisting of hydrogen,alkyl, hydroxyl, alkoxyl, hydroxyalkyl, alkoxyalkyl, alkoxycarbonyl, orcarbamoyl, wherein the carbon containing portions of such group contain1 to 10 carbon atoms; R² is a suitable sulfur protecting group selectedfrom the group consisting of acetyl, benzoyl, methoxyacetyl,1-3-dioxacyclohexyl, 1,3-dioxacyclopentyl, alkoxycarbonyl, carbamoyl,alkoxyalkyl, dialkoxyalkyl, tetrahydropyranyl, tetrahydrofuranyl,p-methoxybenzyl, benzhydryl, trityl, and the like; L is selected fromthe group consisting of ##STR2## wherein k, 1, m and n are 0 to 10,preferably 1 to 6, E is --O--, --S--, or --NR³, wherein R³ is defined inthe same manner as R¹ above, R⁴ is defined in the same manner as R¹above, and X is a suitable coupling moiety selected from the groupconsisting of formyl, carboxyl, hydroxyl, amino, t-butoxycarbonylamino,chlorocarbonyl, N-alkoxycarbamoyl, succinimidyloxycarbonyl, imidate,isocyanate, isothiocyanate, tetrafluorophenoxy, and the like; A isselected from the group consisting of ##STR3## wherein R⁵ to R⁷ aredefined in the same manner as R¹ above, and Y is defined in the samemanner as L above; and B is selected from the group consisting of##STR4## wherein R⁸ and R⁹ are defined in the same manner as R¹ above,and Z is defined in the same manner as L above.

In a preferred embodiment, ligands according to the present inventionhave the general Formula (I) above, wherein A is ##STR5## wherein R⁵ andY are hydrogens; B is ##STR6## wherein R⁸ is hydrogen and Z is a--(CH₂)_(k) --X group wherein k is 4 and X is an amino, or anisothiocyanato group; R² is a benzoyl or a tetrahydropyranyl group; andR¹ and L are hydrogen.

In another preferred embodiment, ligands according to the presentinvention have the general Formula (I) wherein A is ##STR7## wherein R⁵and Y are hydrogens; B is ##STR8## wherein R⁸ is hydrogen and Z is a--(CH₂)_(k) --X group wherein k is 2 and X is either carboxyl orhydroxyl; R² is a benzoyl or a tetrahydropyranyl group; and R¹ and L arehydrogen.

The novel ligands described above, may be incorporated into radionuclidecomplexes used as radiographic imaging agents. Further, these ligands orcomplexes can be covalently or non-covalently attached to biologicallyactive carrier molecules, such as, antibodies, enzymes, peptides,peptidomimetics, hormones, and the like. The complexes of the presentinvention are prepared by reacting one of the aforementioned ligandswith a radionuclide containing solution under radionuclide complexforming reaction conditions. In particular, if a technetium agent isdesired, the reaction is carried out with a pertechnetate solution undertechnetium-99m complex forming reaction conditions. The solvent ifdifferent from water or saline may then be removed by any appropriatemeans, such as evaporation. The complexes are then prepared foradministration to the patient by dissolution or suspension in apharmaceutically acceptable vehicle.

The ligands of the present invention may be prepared from commerciallyavailable starting materials such as 2-aminomethyl imidazole, lysine,homoscrine lactone, glutamic acid, aminoadipic acid, mercaptoaceticacid, etc. by standard synthetic methods as described in the followingExamples 1-2.

Radionuclide complexes according to the present invention may have thegeneral formula: ##STR9## wherein M represents an appropriateradionuclide, such as technetium or rhenium and wherein R¹, A, B, and Lare as defined above in Formula (I).

In a preferred embodiment, a technetium radionuclide complex having thegeneral Formula (II) may be formed from a pertechnetate solution and aligand having the general Formula (I) above, wherein R¹ and L arehydrogen; A is ##STR10## wherein R⁵ and Y are hydrogens; B is ##STR11##wherein R⁸ is hydrogen and Z is a --(CH₂)_(k) --X group wherein k is 4and X is an amino, or an isothiocyanato group.

In another preferred embodiment, a technetium complex having the generalFormula (II) may be formed from a pertechnetate solution and a ligandhaving the general Formula (I) above, wherein R¹ and L are hydrogen; Ais ##STR12## wherein R⁵ and Y are hydrogens; B is ##STR13## wherein R⁸is hydrogen and Z is a --(CH₂)_(k) --X group wherein k is 2 and X iseither carboxyl or hydroxyl.

The radionuclide containing solution may be obtained from radionuclidegenerators in a known manner. For example, when forming a technetiumcomplex, the pertechnetate solution may be obtained from a technetiumgenerator in a known manner. The radionuclide complex forming reactionis then carried out under appropriate reaction conditions. For example,the technetium 99m complex forming reaction is carried out undertechnetium complex forming temperatures, e.g. 20° C. to 100° C. for 10minutes to several hours. The pertechnetate is used in technetiumcomplex forming amounts, e.g. about 10⁻⁶ to 10⁻¹² molar amounts.

The present invention also relates to imaging agents containing aradionuclide complex as described above, in an amount sufficient forimaging, together with a pharmaceutically acceptable radiologicalvehicle. The radiological vehicle should be suitable for injection oraspiration, such as human serum albumin; aqueous buffer solutions, e.g.tris(hydromethyl) aminomethane (and its salts), phosphate, citrate,bicarbonate, etc; sterile water; physiological saline; and balancedionic solutions containing chloride and or dicarbonate salts or normalblood plasma cations such as Ca⁺², Na⁺, K⁺, and Mg⁺².

The concentration of the imaging agent according to the presentinvention in the radiological vehicle should be sufficient to providesatisfactory imaging, for example, when using an aqueous solution, thedosage is about 1.0 to 50 millicuries. The imaging agent should beadministered so as to remain in the patient for about 1 to 3 hours,although both longer and shorter time periods are acceptable. Therefore,convenient ampules containing 1 to 10 mL of aqueous solution may beprepared.

Imaging may be carried out in the normal manner, for example byinjecting a sufficient amount of the imaging composition to provideadequate imaging and then scanning with a suitable machine, such as agamma camera.

The complexes according to the present invention may be prepared inaccordance with the examples set forth below.

EXAMPLE 1 Preparation of3,6-diaza-4-(2-hydroxy)ethyl-1-(S-tetrahydropyranyl)-mercapto-7-(2-imidazolyl)heptane.##STR14##

A mixture of 3-amino-5-aza-6-(2-imidazolyl)-4-oxohexan-1-ol (1.98 g,0.01 mol) and (S-tetrahydropyranyl)mercaptoscetic acid N-succinimidoester (2.63 g, 0.01 mol) in acetonitrile (20 mL) was stirred at ambienttemperature for 4 hours. The solvent was evaporated in vacuo and theresidue was purified by flash chromatography over reverse phase (50 g).Elution with water removed N-hydroxy succinimide along with lowmolecular weight impurities. Elution with water/methanol (3:2) followedby evaporation of the solvent furnished the desired ligand as pale tanfoam. ¹³ C-NMR (D₂ O) δ 173.4, 172.7, 143.9, 121.1, 82.0, 65.2, 56.9,50.7, 35.4, 32.6, 29.7, 28.2, 23.8, 20.4; thermal spray mass spectrum,m/Z 357 (M+1).

EXAMPLE 2 Preparation of technetium-99m complex of the ligand inExample 1. ##STR15##

A solution of the ligand in Example 1 (110 μL of 1.0 mg/mL stocksolution in 0.5M sodium phosphate buffer, pH 8.0) was placed in a vialcontaining sodium gluconate (50 mg), and stannous chloride (0.5 mg). Asolution of sodium pertechnetate in saline (1 mL) was then added and theentire mixture was heated in boiling water bath for 15 minutes. Theproduct was purifed by reverse phase HPLC to give neutral ^(99m) Tc(V)complex in about 50-60% yield.

The foregoing has been a discussion of the preferred embodiments of thepresent invention, but is not intended to limit the invention in anyway. Rather, many modifications, variations and changes in detail may bemade within the scope of the present invention.

What is claimed is:
 1. A radionuclide complex having the generalformula: ##STR16## wherein M represents an appropriate radionuclide; R¹is selected from the group consisting of hydrogen, alkyl, hydroxyl,alkoxyl, hydroxyalkyl, alkoxyalkyl, alkoxycarbonyl, or carbamoyl,wherein the carbon containing portions of such group contain 1 to 10carbon atoms; L is selected from the group consisting of ##STR17##wherein k, l, m and n are 0 to 10, E is --O--, --S--, or --NR³, whereinR³ is defined in the same manner as R¹ above, R⁴ is defined in the samemanner as R¹ above, and X is a suitable coupling moiety selected fromthe group consisting of formyl, carboxyl, hydroxyl, amino,t-butoxycarbonylamino, chlorocarbonyl, N-alkoxycarbamoyl,succinimidyloxycarbonyl, imidate, isocyanate, isothiocyanate,tetrafluorophenoxy, and the like; A is selected from the groupconsisting of ##STR18## wherein R⁵ to R⁷ are defined in the same manneras R¹ above, and Y is defined in the same manner as L above; and B isselected from the group consisting of ##STR19## wherein R⁸ and R⁹ aredefined in the same manner as R¹ above, and Z is defined in the samemanner as L above.
 2. A complex according to claim 1, wherein M istechnetium or rhenium.
 3. A radiographic imaging agent comprising acomplex having the general formula: ##STR20## wherein M represents anappropriate radionuclide; wherein R¹ is hydrogen; L is hydrogen; A is##STR21## wherein R⁵ and Y are hydrogen; and B is ##STR22## wherein R⁸and Z are hydrogen; and a pharmaceutically acceptable vehicle.
 4. Animaging agent according to claim 3, wherein said vehicle is suitable forinjection or aspiration and is selected from the group consisting ofhuman serum albumin, aqueous buffer solutions, sterile water,physiological saline, and balance ionic solutions containing chloridesalts, dicarbonate salts or blood plasma cations.
 5. An imaging agentaccording to claim 3, wherein M is technetium or rhenium.
 6. An imagingagent according to claim 3, wherein the concentration of said complex insaid vehicle is from about 1.0 to 50 millicuries.
 7. A method ofradiographic imaging, comprising injecting a sufficient amount ofimaging agent to provide imaging and then scanning with a scanningmachine; said imaging agent comprising a complex having the generalformula: ##STR23## wherein M represents an appropriate radionuclide;wherein R¹ is hydrogen; L is hydrogen; A is ##STR24## wherein R⁵ and Yare hydrogen; and B is ##STR25## wherein R⁸ and Z are hydrogen.
 8. Amethod according to claim 7, wherein M is technetium or rhenium.